extra dns zone files for backup zone?

[dfjkl]

Is it normal to see files like this:

-rw-r--r--   1 root  wheel  2189 Sep 19 17:37 zone.org.bak
-rw-r--r--   1 root  wheel     0 May  1 04:23 zone.org.bak.6dKaRF
-rw-r--r--   1 root  wheel     0 Sep 17 20:22 zone.org.bak.8u8Y1n
-rw-r--r--   1 root  wheel     0 Aug 19 17:39 zone.org.bak.ZxKtb7

...for a backup slave zone?  These are sitting on a backup ns server that I
haven't examined in a little while.

Thanks,
Frank